With the vast majority of airlines (and certainly all of the budget carriers), there's no such thing as a return fare. If, for example, you book with Ryanair from Stansted to Barcelona and back, you'll be quoted a total fare which is simply the sum of the two single fares.
So it's unsurprising that cheap fares are always advertised as 'singles'. As you suggest, there are certainly times when it's possible to only find cheap fares in one direction (with 'mega-buck' fares in the opposite direction) but it's by no means the norm.
I live about an hour's drive from Stansted and I like to take day trips with Ryanair. If I'm only going to be abroad for, say, 12 hours I'm only prepared to pay the absolute minimal fares. I've been to Barcelona (twice), Rome (three times), Genoa (twice), Malmo, Brussels, Bergamo, Dublin, Frankfurt (three times) and other places as well. I can't recall ever paying much more than £20 total for my outward and return flights, including all taxes and charges. (Many flights have only cost a total of a tenner)
Those cheap fares aren't just there for day trips. If I'd have been willing to pay for accommodation, I could have stayed away for a short break or a week or a fortnight.
To find the cheapest fare it helps to be flexible with your destination. (It's usually far easier to find cheap flights to destinations such as Frankfurt, rather than to the 'sunshine hotspots', but there are still some cheap flights to the more popular destinations). Then it helps if you can fly at 'unsocial' times. (Catching a really early flight in the morning outbound, and a really late one at night is ideal for my day trips, but it also tends to get the cheapest fares). Lastly it helps if you can fly midweek. (Holidaymakers like weekend flights. Business people like flights on Mondays and Fridays. Tuesday, Wednesday and Thursday are usually cheapest).
Chris